In modern civil engineering, the use of gantries (overslung and underslung) in the construction of bridges and viaducts has largely overtaken ground resting scaffolding. However, the factor that hinders their more generalized use is the fact that they represent a large investment in terms of materials and construction labour. Although present gantries are reusable, it is also very common for them to require re-adaptation, in particular when the project calls for it to carry more load then it was initially designed for. This adaptation is in itself a very time consuming and expensive process which normally delays the pace of construction.
The use of prior art gantries also implies some considerable risk. This is because they are structures that are meant to support a great deal of permanent and variable loads and can lead to a great deal of deformations and stresses that weaken the structure and may eventually lead to its collapse. Several accidents have occurred in the past.
The use of cables or tendons with adjustable prestressing have been used in the past to strengthen and reinforce concrete girders as can be seen in the patent applications WO 00/68508 (Interconstec Co. Ltd), WO 02/28168 (Interconstec Co. Ltd) and WO 01/27406. However, these structures required the introduction of external tools to increase or decrease the tension of the cables. The adjustment was also not made in response so the loads applied to the structure at any given time, but was rather included in a strategy of periodic maintenance of the girders.